1) Field of the Disclosure
The disclosure relates generally to composite structures and methods of forming the same, and more specifically, to composite structures having reduced area radius fillers and methods of forming the same, such as for stringer composite structures in aircraft wings.
2) Description of Related Art
Composite structures are used in a wide variety of applications, including in the manufacture of aircraft, spacecraft, rotorcraft, watercraft, automobiles, trucks, and other vehicles and structures, due to their high strength-to-weight ratios, corrosion resistance, and other favorable properties. In aircraft construction, composites structures are used in increasing quantities to form the wings, fuselage, tail sections, and other components.
For example, aircraft wings may be formed of composite stiffened panel structures comprising composite skin panels or webs to which reinforcing stiffeners or “stringers” may be attached or bonded to improve the strength, stiffness, buckling resistance, and stability of the composite skin panels or webs. The stringers attached or bonded to the composite skin panels or webs may be configured to carry various loads and may be provided in a variety of different cross-sectional shapes, such as I-beams, T-stiffeners, and J-stiffeners.
Known stringers found in aircraft composite wing structures may have a low pull-off strength. Consequently, such stringers may not be loaded through a stringer blade portion. This may require that holes be drilled in the wing skin and that fasteners be attached through the wing skin to attach, for example, wing rib fittings to the wing skin. However, this may create additional areas on the aircraft subject to possible fuel leaks or manufacturing issues and complications.
Moreover, such fasteners may need to be treated and triple protected for lightening strike protection, and such fastener holes may require liquid tight sealing so that they are not subject to fuel leaks. For example, such fasteners protruding into a fuel cell in the wing may need to be countersunk, coated on the outside with an insulating plug, coated on the inside with an insulating sealant, and grounded to prevent sparking inside of the fuel cell. The time required for installing such fasteners may be increased, which, in turn, may increase manufacturing complexity and cost. In addition, the presence of additional fasteners may add weight to the aircraft, which, in turn, may reduce the payload capacity of the aircraft and may increase fuel consumption, which may result in increased fuel costs.
Void regions may be formed by the radius of curved portions of the stringers when they are attached or joined perpendicularly to composite skin panels or webs. Such void regions may typically be referred to as “radius filler regions” or “noodles”. Such radius filler regions or noodles within stringers may be prone to cracking because they may be three-dimensionally constrained. Radius fillers or noodles made of composite material or adhesive/epoxy material and having a generally triangular cross-section may be used to fill the radius filler regions or noodles in order to provide additional structural reinforcement to such regions. However, known radius fillers or noodles may be made of a material that is different from or not compatible with the material of the composite structure surrounding the radius filler or noodle. This may result in different material properties which may, in turn, require modifications to cure cycles, processing temperatures and pressures, and/or relative amounts of fibers and resin matrices. Such modifications may increase manufacturing time, labor and costs.
A difference in coefficients of thermal expansion (CTE) of the radius filler or noodle material and the material of the composite structure surrounding the radius filler or noodle may cause the radius filler or noodle to be susceptible to thermal cracking. In addition, known unidirectional tape radius fillers or noodles may be susceptible to thermal cracking after curing, if a stiffener cross-sectional area becomes very large. For example, known designs using one large radius filler or noodle may be susceptible to cracking due to increased CTE differences between the large radius filler and the surrounding laminate structure.
To prevent such known unidirectional tape radius fillers or noodles from thermal cracking, the unidirectional tape radius fillers or noodles may be wrapped in fabric to prevent the thermal cracking from spreading to surrounding structures. However, such fabric may need to be applied manually to the surrounding structure, such as the stringer, and this may result in additional manufacturing time, labor, and costs, as well as an increase in possible errors.
Further, known unidirectional/laminate radius fillers or noodles may have relatively blunt tips on the three corners of the radius filler or noodle. A zero degree (0°) ply of pre-preg (i.e., reinforcement fibers impregnated with a resin material) may be folded over itself repeatedly to form a circular radius filler or noodle. The radius filler or noodle may then be formed into a triangular shape under heat and vacuum. The blunt noodle tip may create resin rich pockets at the tips of the radius filler or noodle, and such regions may be susceptible to initiation of crack propagation. The crack may spread between composite plies and the crack may cause premature stringer pull-off strength issues. A low pull-off strength may prevent the stringers from being used as structural attachment points inside the wing box. This, in turn, may require, as discussed above, that holes be drilled in the wing skin and that fasteners be attached through the wing skin to attach wing rib fittings to the wing skin.
Accordingly, there is a need in the art for composite structures having reduced area radius fillers and methods of forming the same that provide advantages over known structures and methods.